The role of surface wettability on bubble formation in air-water systems.

The production of microbubbles is rapidly becoming of considerable global importance with many industries taking advantage of the increased mass transfer rates the bubbles can attain. Many factors have interrelated roles during bubble formation, with effects such as gas flow rate, liquid viscosity, pore size and pore orientation all imparting considerable influence during the formation process. Many of these features have been examined in detail and are relatively well understood. However, the role of surface wettability and the interactions at the gas-liquid-solid triple interface have for the most part been neglected, and it is the role of this wettability that is examined herein.
Utilising the well-studied wet chemistry surface modification techniques of silanes and thiols, many substrates have been modified and the wettability tested. Contact angle goniometry has been utilised to assess the wetting characteristics of each substrate, and the role of surface roughness has been discussed in relation to both the static Young’s contact angle and the advancing and receding angles.
Modified porous plates have been used to generate bubbles, with controlled single pore, multiple controlled pore, and multiple randomised pore systems being investigated. A steady flow of air was bubbled into distilled water through the various diffuser plates. It has been observed a contact angle of 90° is of vital importance, with a significant increase of bubble size above the 90° angle, defined as the hydrophobic wetting region. On the contrary, bubble size is greatly reduced below this angle, in the region defined as the hydrophilic region. The effect is seen to increase as the density of pores increases when the plate from which they are emitted is relatively smooth. Upon roughening, the effect is seen to diminish, and mechanisms for this process have been postulated. It is thought that the surface topography disrupts the modifying layers and also physically restricts the growing bubble, preventing the growth of the bubbles emitted from a hydrophobic surface. Attempts have been made to support this hypothesis both qualitatively and quantitatively.
The fluidic oscillator of Zimmerman and Tesar has been examined, with numerous physical features being investigated. The oscillator was then added to the system to investigate the effect of wettability under substantial oscillation. It has been shown that the bubble size emitted from hydrophobic surfaces is significantly reduced when compared to the steady flow system. The effect is believed to be due to the ‘suction’ component of the oscillatory flow created by the oscillator. It has been seen via high speed photography that the growth rate of the growing bubble slows significantly as the flow begins to switch, before a reduction in size is seen as the gas is removed from the bubble. The opposing forces of buoyancy and suction act to elongate the bubble neck causing break off at a significantly reduced size. Although the diffuser plate is often observed to oscillate like the skin of a drum, this is not the predominant cause of the size reduction. Further experiments have been conducted using a synthetic actuator jet to create a pulsed air flow with only a positive component. Bubble size is not affected in this case, despite frequency sweeps being employed.